1. Field of the Invention
The present invention relates to a holographic recording medium and, more particularly, to a holographic recording medium in which a fixing process is performed by applying bleaching light in order to fix holographic recorded data in the medium.
2. Description of the Related Art
As a medium in which large-capacity information can be recorded at high density, there is a holographic recording medium. In the holographic recording medium, page data having capacity of several hundreds of megabytes can multiplex-recorded in the same region. The holographic recording is such that a light beam from a single light source is separated to reference light and information light and the same position on the recording medium is irradiated with the reference light and the information light and an irradiation angle or a wavelength of the reference light is varied to generate different interferences, so that different information is recorded in the same position on the recording medium in piles.
The region in which information is recorded by the holographic recording is a region having a three-dimensional thickness and its optical characteristics are varied even when weak light such as a fluorescent lamp in a room is applied, depending on its material. When the optical characteristics are varied, a reproduction error could be generated. Thus, a medium in which a holographic recording medium is housed in a cartridge or a medium in which a light shielding layer is provided on a holographic recording layer have been proposed (refer to Japanese Unexamined Patent Publication No. 2004-279942).
In addition, according to a holographic recording medium in which data can be written only one time such as a CD-R, in order to stabilize the optical characteristics, after data is recorded with hologram, data fixing process called bleaching is performed. The bleaching is a process such that a region in which holographic recording has been performed is irradiated with white light (referred to as bleaching light) having relatively high intensity which does not destroy its recorded data.
In the recording region irradiated with the bleaching light, reactivity of the recording material is fixed and its optical characteristics are not varied by weak light such as a fluorescent lamp, so that the recorded data can be stably reproduced.
FIGS. 21 and 22 show a schematic constitution of a conventional holographic recording medium. FIG. 21 is a sectional view showing the medium and FIG. 22 is a plan view showing a substrate constitution.
As shown in FIGS. 21 and 22, the holographic recording medium has a constitution in which a resin substrate 12 and a holographic material layer 13 are sandwiched between two glass substrates 10 (10-1, 10-2). The resin substrate 12 includes a reflection film 11 at a boundary with one of the glass substrates 10-1.
When the reference light and the information light are applied at the same time from an upper part of the glass substrate 10-2 of the medium, data is recorded in a region of the holographic material layer 13. After the data is recorded, the bleaching light is applied to fix the recorded data. Thus, the data is fixed in the region of the holographic material layer 13 irradiated with the bleaching light.
FIGS. 23A and 23B show a schematic explanatory diagram of the conventional medium when the bleaching light is applied.
As shown in FIGS. 23A and 23B, when the bleaching light 15 proceeds into the recording region of the holographic material layer 13, it is diffused and diffracted. The bleaching light proceeds to its adjacent region 17 by diffusion and diffraction besides a light beam proceeding to the intended irradiation region 16. As a result, data recorded in the regions (16 and 17) to which the bleaching light 15 proceeded is fixed.
However, when the data is fixed by the bleaching light in the conventional holographic medium, there are following problems.
As shown in FIG. 24, it is assumed that a holographic recording completed region 18 and an unrecorded region 19 are adjacent to each other in the holographic material layer 13. Here, according to the holographic recording completed region 18, it is assumed that data recording with the information light and the reference light has been completed but the bleaching light is not applied yet.
According to the unrecorded region 19, it is assumed that data has not been recorded yet.
As shown in FIG. 25, it is assumed that the bleaching light 15 is applied to the medium in this state to fix the data of the recording completed region 18. At this time, the data can be fixed as intended in the recording completed region 18 which is apart from the unrecorded region 19. However, when the bleaching light 15 is applied to a boundary region with the region 19, since the holographic material layer 13 has a thickness in the vertical direction, it is diffused and diffracted as shown in FIGS. 23A and 23B, so that the bleaching light proceeds beyond a boundary 20. That is, the bleaching light 15 proceeds to regions 16 and 17 and the region 17 irradiated with the bleaching light is spread to the left side of the boundary 20 in FIG. 25.
As a result, as shown in FIG. 26, the data in the unrecorded region 19 in the vicinity of the boundary is also fixed in addition to the data of the intended recording completed region 18. A region 21 shown in FIG. 26 is the data fixed region. That is, although the region 19 is to be used for recording data in the future, since unintended fixing process was performed, the holographic recording cannot be performed any more in that region 19.
Alternatively, in the fixed region in the unrecorded region 19, its diffraction efficiency is extremely lowered and even if the recording and reproducing can be performed thereafter, SNR at the time of recording and reproducing deteriorates.
As described above, when the bleaching process is performed in the conventional holographic recording medium, recording and reproducing characteristics deteriorate and recording capacity is reduced in the boundary region of the recording completed region.
In addition, data is managed by logical and physical addresses in a conventional storage. Since the logical address and the physical address are related at an initial stage, when the unrecorded region becomes a region in which data cannot be recorded, performance is considerably lowered because of its recording error or frequent occurrence of alternating processes. Thus, in the case of the medium in which recording capacity is varied, it is necessary to develop a data management method which is different from the conventional method.